Dilute acid pretreatment and enzymatic saccharification of sugarcane tops for bioethanol production

The aim of this work was to study the feasibility of using sugarcane tops as feedstock for the production of bioethanol. The process involved the pretreatment using acid followed by enzymatic saccharification using cellulases and the process was optimized for various parameters such as biomass loading, enzyme loading, surfactant concentration and incubation time using Box–Behnken design. Under optimum hydrolysis conditions, 0.685 g/g of reducing sugar was produced per gram of pretreated biomass. The fermentation of the hydrolyzate using Saccharomyces cerevisae produced 11.365 g/L of bioethanol with an efficiency of about 50%. This is the first report on utilization of sugarcane tops for bioethanol production.     

Subcritical water and dilute acid pretreatments for bioethanol production from Melaleuca leucadendron shedding bark

The feasibility of bioethanol production using the lignocellulose of the shedding bark of Melaleuca leucadendron (Paper bark tree) was investigated. The effects of pretreatment parameters (temperature, time and acid concentration) on the yields of sugars and inhibitors, and optimal pretreatment conditions were determined. At very low severity conditions (combined severity factor, CSF ≤ 0.335), 28% of xylan was recovered and this recovery increased with increasing CSF till it peaked to 64.4% (11.2 g xylose L⁻¹) at a CSF of 1.475. However, at CSF > 2.0, xylose yield declined due to degradation. Mild and progressive glucose yield was detected in prehydrolysate at CSF ≥ 1.514, and subsequent enzymatic hydrolysis allowed complete glucan solubilization. Implementing environmentally friendly subcritical water pretreatment at CSF ≤ 0.335 on the shedding bark, about 85% of glucan solubilization was achieved after enzymatic hydrolysis. An industrial Saccharomyces cerevisiae strain readily fermented crude hydrolysate within 12 h, yielding 24.7 g L⁻¹ ethanol at an inoculum size of 2% (v/v), representing a glucose to ethanol conversion rate of 0.475 g g⁻¹ (91% ethanol yield). Based on our findings, the shedding bark is a potential feedstock for bio-ethanol production.     

Evaluation of steam explosion pre-treatment for enzymatic hydrolysis of sunflower stalks

Sunflower stalks, a largely available and cheap agricultural residue lacking of economic alternatives, were subjected to steam explosion pre-treatment, the objective being to optimize pre-treatment temperature in the range 180-230°C. Enzymatic hydrolysis performed on the pre-treated solids by a cellulolytic complex (Celluclast 1.5L) and analysis of filtrates were used to select the best pre-treatment temperature. Temperature selection was based on the susceptibility to enzymatic hydrolysis of the cellulose residue and both the cellulose recovery in the solid and the hemicellulose-derived sugars recoveries in the filtrate. After 96h of enzymatic action, a maximum hydrolysis yield of 72% was attained in the water-insoluble fiber obtained after pre-treatment at 220°C, corresponding to a glucose concentration of 43.7g/L in hydrolysis media. Taking into account both cellulose recovery and hydrolysis yield, the maximum value of glucose yield referred to unpretreated raw material was also found when using steam pre-treated sunflower stalks at 220°C, obtaining 16.7g of glucose from 100g of raw material. With regard to the filtrate analysis, most of the hemicellulosic-derived sugars released during the steam pre-treatment were in oligomeric form, the highest recovery being obtained at 210°C pre-treatment temperature. Moreover, the utilisation of hemicellulosic-derived sugars as a fermentation substrate would improve the overall bioconversion of sunflower stalks into fuel ethanol.     

Potential of termite-based biomass pre-treatment strategies for use in bioethanol production

Abstract When considering the current state of the biorefinery industry, it is readily apparent that industrial cellulose and hemicellulose digestion processes are relatively advanced, whereas enzymatic pre-treatment strategies for biomass delignification and cellulose solubilization are not well developed. The need for efficient biomass pre-treatment strategies presents a significant opportunity for researchers studying lignocellulose digestion in termites and other insects. With an emphasis on industrial biomass pre-treatment, this review provides an overview of: (i) industrial biorefining operations (feedstocks, processing, and economics); (ii) recent findings from termite research that have revealed candidate enzymes; and (iii) research needs and opportunities for consideration by entomologists working in this area. With respect to research findings, recently identified candidate lignases (laccases, catalases, peroxidases, esterases), other potentially important detoxification enzymes (cytochrome P450, superoxide dismutase), and phenolic acid esterases (carboxylesterases) that may assist in hemicellulose solubilization are overviewed. Regarding research needs and opportunities, several approaches for identification of candidate pre-treatment enzymes from upstream, symbiont-free gut regions are also described.     

Alkaline pre-treatment of oilseed rape straw for bioethanol production: evaluation of glucose yield and pre-treatment energy consumption

The objective of the research was to investigate the effect of biomass loading, alkali (NaOH) concentration and pre-treatment time on the yield of glucose obtained following alkaline pre-treatment and enzymatic hydrolysis of oilseed rape (OSR) straw. A maximum glucose yield of (440.6 ± 14.9) g glucose kg⁻¹ biomass was obtained when OSR straw was pre-treated at a biomass loading of 50 g kg⁻¹ and an alkali concentration of 0.63 mol dm⁻³ NaOH for 30 min. The energy efficiency of glucose extraction (0.39 kg glucose MJ⁻¹ consumed) was highest when OSR straw was pre-treated at a biomass loading of 50 g kg⁻¹ and an alkali concentration of 0.63 or 0.75 mol dm⁻³ for 30 min. The study demonstrated alkaline pre-treatment of OSR straw is superior to acid pre-treatment in terms of glucose yield and energy efficiency.     

Evaluation of the overall process on bioethanol production from miscanthus hydrolysates obtained by dilute acid pretreatment

In this study, dilute sulfuric acid pretreatment was performed to improve the sugars recovery from Korean Miscanthus straw. The effect of pretreatment conditions on solubilized xylose was fundamentally investigated for the efficient removal of xylan. The optimal conditions were determined using a statistical method, and were shown to be a temperature of 121.6°C, an acid concentration of 1.1%, and a reaction time of 12.8 min. The combined severity factor was shown to be 1.1 under the optimum conditions. Following the pretreatment, the solubilized xylose in liquid fraction was found to be 71.2%, and about 72.6% of the solid was recovered. After enzymatic hydrolysis, about 86.4% glucose conversion was achieved when the pretreated biomass was used as a substrate, with the conversion being improved 4-fold compared with the control (untreated). The hydrolysates, approximately 10 g/L glucose, were applied to the fermentation of Saccharomyces cerevisiae K35, and the ethanol yield was about 96%. The overall process was evaluated based on the material balance, and the results show that approximately 172 g bioethanol can be produced when 1,000 g Miscanthus straw is loaded into the process.     

Organosolvent pretreatment and enzymatic hydrolysis of rice straw for the production of bioethanol

The present study investigates the operational conditions for organosolvent pretreatment and hydrolysis of rice straw. Among the different organic acids and organic solvents tested, acetone was found to be most effective based on the fermentable sugar yield. Optimization of process parameters for acetone pretreatment were carried out. The structural changes before and after pretreatment were investigated by scanning electron microscopy, X-ray diffraction and Fourier transform infrared (FTIR) analysis. The X-ray diffraction profile showed that the degree of crystallinity was higher for acetone pretreated biomass than that of the native. FTIR spectrum also exhibited significant difference between the native and pretreated samples. Under optimum pretreatment conditions 0.458 g of reducing sugar was produced per gram of pretreated biomass with a fermentation efficiency of 39%. Optimization of process parameters for hydrolysis such as biomass loading, enzyme loading, surfactant concentration and incubation time was done using Box–Benhken design. The results indicate that acetone pretreated rice straw can be used as a good feed stock for bioethanol production.     

Enhanced enzymatic hydrolysis of rapeseed straw by popping pretreatment for bioethanol production

The objective of this study was to find a pretreatment process that enhances enzymatic conversion of biomass to sugars. Rapeseed straw was pretreated by two processes: a wet process involving wet milling plus a popping treatment, and a dry process involving popping plus dry milling. The effects of the pretreatments were studied both in terms of structural and compositional changes and change in susceptibility to enzymatic hydrolysis. After application of the wet and dry processes, the amounts of cellulose and xylose in the straw were 37-38% and 14-15%, respectively, compared to 31% and 12% in untreated counterparts. In enzymatic hydrolysis performance, the wet process presented the best glucose yield, with a 93.1% conversion, while the dry process yielded 69.6%, and the un-pretreated process yielded <20%. Electron microscopic studies of the straw also showed a relative increase in susceptibility to enzymatic hydrolysis with pretreatment.     

Sustainable bioethanol production combining biorefinery principles using combined raw materials from wheat undersown with clover-grass

To obtain the best possible net energy balance of the bioethanol production the biomass raw materials used need to be produced with limited use of non-renewable fossil fuels. Intercropping strategies are known to maximize growth and productivity by including more than one species in the crop stand, very often with legumes as one of the components. In the present study clover-grass is undersown in a traditional wheat crop. Thereby, it is possible to increase input of symbiotic fixation of atmospheric nitrogen into the cropping systems and reduce the need for fertilizer applications. Furthermore, when using such wheat and clover-grass mixtures as raw material, addition of urea and other fermentation nutrients produced from fossil fuels can be reduced in the whole ethanol manufacturing chain. Using second generation ethanol technology mixtures of relative proportions of wheat straw and clover-grass (15:85, 50:50, and 85:15) were pretreated by wet oxidation. The results showed that supplementing wheat straw with clover-grass had a positive effect on the ethanol yield in simultaneous saccharification and fermentation experiments, and the effect was more pronounced in inhibitory substrates. The highest ethanol yield (80% of theoretical) was obtained in the experiment with high fraction (85%) of clover-grass. In order to improve the sugar recovery of clover-grass, it should be separated into a green juice (containing free sugars, fructan, amino acids, vitamins and soluble minerals) for direct fermentation and a fibre pulp for pretreatment together with wheat straw. Based on the obtained results a decentralized biorefinery concept for production of biofuel is suggested emphasizing sustainability, localness, and recycling principles. JIMB 2008: BioEnergy—Special issue.     

Microwave as a pre-treatment of triticale for bioethanol fermentation and utilization of the stillage for lactic acid fermentation

The aims of this research were to investigate the potential for improvement of bioethanol fermentation by microwave pre-treatment of triticale, as well as the possibility of using triticale stillage (a by-product of bioethanol fermentation) as a suitable medium for lactic acid fermentation, either without or with the optimal addition of CaCO₃. The results showed that an adequate microwave pre-treatment (270 and 450 W) marginally improved bioethanol production, giving a slightly higher yield of the bioethanol produced (by 3.95%). Lactobacillus fermentum PL-1 was used for subsequent lactic acid fermentation of the stillage that remained after bioethanol fermentation. Results showed higher pH decrease and higher acidity in the samples without the addition of CaCO₃. Consequently, higher concentrations of l- and d-lactic acid were determined in the samples with the addition of CaCO₃, as well as better viability of cells and the complete glucose utilization.     

Optimization of enzymatic hydrolysis and fermentation conditions for improved bioethanol production from potato peel residues

The aim of this work was the optimization of the enzyme hydrolysis of potato peel residues (PPR) for bioethanol production. The process included a pretreatment step followed by an enzyme hydrolysis using crude enzyme system composed of cellulase, amylase and hemicellulase, produced by a mixed culture of Aspergillus niger and Trichoderma reesei. Hydrothermal, alkali and acid pretreatments were considered with regards to the enhancement of enzyme hydrolysis of potato peel residues. The obtained results showed that hydrothermal pretreatment lead to a higher enzyme hydrolysis yield compared to both acid and alkali pretreatments. Enzyme hydrolysis was also optimized for parameters such as temperature, pH, substrate loading and surfactant loading using a response surface methodology. Under optimized conditions, 77 g L^?1 of reducing sugars were obtained. Yeast fermentation of the released reducing sugars led to an ethanol titer of 30 g L^?1 after supplementation of the culture medium with ammonium sulfate. Moreover, a comparative study between acid and enzyme hydrolysis of potato peel residues was investigated. Results showed that enzyme hydrolysis offers higher yield of bioethanol production than acid hydrolysis. These results highlight the potential of second generation bioethanol production from potato peel residues treated with onsite produced hydrolytic enzymes. © 2017 American Institute of Chemical Engineers Biotechnol. Prog., 33:397–406, 2017     

Pretreatment of Miscanthus with biomass-degrading bacteria for increasing delignification and enzymatic hydrolysability

Biomass recalcitrance is still a main challenge for the production of biofuels and high-value products. Here, an alternative Miscanthus pretreatment method by using lignin-degrading bacteria was developed. Six efficient Miscanthus-degrading bacteria were first cultured to produce laccase by using 0.5% Miscanthus biomass as carbon source. After 1–5 days of incubation, the maximum laccase activities induced by Miscanthus in the six strains were ranged from 103 to 8091 U l⁻¹. Then, the crude enzymes were directly diluted by equal volumes of citrate buffer and added Miscanthus biomass to a solid concentration at 4% (w/v). The results showed that all bacterial pretreatments significantly decreased the lignin content, especially in the presence of two laccase mediators (ABTS and HBT). The lignin removal directly correlated with increases in total sugar and glucose yields after enzymatic hydrolysis. When ABTS was used as a mediator, the best lignin-degrading bacteria (Pseudomonas sp. AS1) can remove up to 50.1% lignin of Miscanthus by obtaining 2.2-fold glucose yield, compared with that of untreated biomass. Therefore, this study provided an effective Miscanthus pretreatment method by using lignin-degrading bacteria, which may be potentially used in improving enzymatic hydrolysability of biomass.     

Pretreatment of waste newspaper using ethylene glycol for bioethanol production

Pretreatment using ethylene glycol was investigated to enhance the enzyme digestibility of wastepaper. The pretreatment was conducted over a wide range of conditions including sulfuric acid concentrations of 1.3 ∼ 4.7%, temperatures of 143.2 ∼ 176.7°C and reaction time of 1.6 ∼ 18.4 min. The optimum conditions were around 2% sulfuric acid, 150°C and 15 min. At these conditions, 60 and 75% of hemicellulose and lignin, respectively, were removed while cellulose remained intact. Additionally, an enzyme digestibility of 94% was achieved. From the substrate dissolution analysis, the dissolution yield was strongly related to the enzymatic digestibility and the removal of cellulose, hemicellulose and lignin. Also, the dissolution yield was directly related to the severity parameter, which provides a quantitative prediction of the intensity of a reaction. Recycling and re-use of ethylene glycol were also studied. Ethylene glycol could be recycled and re-used at least four times without significantly lowering the pretreatment performance.     

Utilization of by-products derived from bioethanol production process for cost-effective production of lactic acid

The by-products of bioethanol production such as thin stillage (TS) and condensed distillers solubles (CDS) were used as a potential nitrogen source for economical production of lactic acid. The effect of those by-products and their concentrations on lactic acid fermentation were investigated using Lactobacillus paracasei CHB2121. Approximately, 6.7 g/L of yeast extract at a carbon source to nitrogen source ratio of 15 was required to produce 90 g/L of lactic acid in the medium containing 100 g/L of glucose. Batch fermentation of TS medium resulted in 90 g/L of lactic acid after 48 h, and the medium containing 10 % CDS resulted in 95 g/L of lactic acid after 44 h. Therefore, TS and CDS could be considered as potential alternative fermentation medium for the economical production of lactic acid. Furthermore, lactic acid fermentation was performed using only cassava and CDS for commercial production of lactic acid. The volumetric productivity of lactic acid [2.94 g/(L·h)] was 37 % higher than the productivity obtained from the medium with glucose and CDS.     

High temperature dilute acid pretreatment of coastal Bermuda grass for enzymatic hydrolysis

Dilute sulfuric acid was used to pretreat coastal Bermuda grass at high temperature prior to enzymatic hydrolysis. After both pretreatment and enzymatic hydrolysis processes, the highest yield of total sugars (combined xylose and glucose) was 97% of the theoretical value. The prehydrolyzate liquor was analyzed for inhibitory compounds (furfural, hydroxymethylfurfural (HMF)) in order to assess potential risk for inhibition during the following fermentation. Accounting for the formation of the inhibitory compounds, a pretreatment with 1.2% acid at 140 °C for 30 min with a total sugar yield of 94% of the theoretical value may be more favorable for fermentation. From this study, it can be concluded that dilute sulfuric acid pretreatment can be successfully applied to coastal Bermuda grass to achieve high yields of monomeric glucose and xylose with acceptable levels of inhibitory compound formation.     

Valorization of olive stones for xylitol and ethanol production from dilute acid pretreatment via enzymatic hydrolysis and fermentation by Pachysolen tannophilus

Olive stones are an agro-industrial by-product abundant in the Mediterranean area that is regarded as a potential lignocellulosic feedstock for sugar production. Statistical modeling of dilute-sulphuric acid hydrolysis of olive stones has been performed using a response surface methodology, with treatment temperature and process time as factors, to optimize the hydrolysis conditions aiming to attain maximum d-xylose extraction from hemicelluloses. Thus, solid yield and composition of solid and liquid phases were assessed by empirical modeling. The highest yield of d-xylose was found at a temperature of 195 °C for 5 min. Under these conditions, 89.7% of the total d-xylose was recovered from raw material. The resulting solids from optimal conditions were assayed as substrate for enzymatic hydrolysis, while fermentability of hemicellulosic hydrolysates was tested using the d-xylose-fermenting yeast Pachysolen tannophilus. Both bioprocesses were considerably influenced by enzyme loading and inoculum size. In the enzymatic hydrolysis step, about 56% of cellulose was converted into d-glucose by using an enzyme/solid ratio of 40 FPU g⁻¹, while in the fermentation carried out with a cell concentration of 2 g L⁻¹ a yield of 0.44 g xylitol/g d-xylose and a global volumetric productivity of 0.11 g L⁻¹ h⁻¹ were achieved.     

Simultaneous saccharification and fermentation of lignocellulosic residues pretreated with phosphoric acid–acetone for bioethanol production

Bermudagrass, reed and rapeseed were pretreated with phosphoric acid–acetone and used for ethanol production by means of simultaneous saccharification and fermentation (SSF) with a batch and fed-batch mode. When the batch SSF experiments were conducted in a 3% low effective cellulose, about 16 g/L of ethanol were obtained after 96 h of fermentation. When batch SSF experiments were conducted with a higher cellulose content (10% effective cellulose for reed and bermudagrass and 5% for rapeseed), higher ethanol concentrations and yields (of more than 93%) were obtained. The fed-batch SSF strategy was adopted to increase the ethanol concentration further. When a higher water-insoluble solid (up to 36%) was applied, the ethanol concentration reached 56 g/L of an inhibitory concentration of the yeast strain used in this study at 38 °C. The results show that the pretreated materials can be used as good feedstocks for bioethanol production, and that the phosphoric acid–acetone pretreatment can effectively yield a higher ethanol concentration.     

Deciphering role of technical bioprocess parameters for bioethanol production using microalgae

Microalgae biomass is considered an important feedstock for biofuels and other bioactive compounds due to its faster growth rate, high biomass production and high biomolecules accumulation over first and second-generation feedstock. This research aimed to maximize the specific growth rate of fresh water green microalgae Closteriopsis acicularis, a member of family Chlorellaceae under the effect of pH and phosphate concentration to attain enhanced biomass productivity. This study investigates the individual and cumulative effect of phosphate concentration and pH on specific growth characteristics of Closteriopsis acicularis in autotrophic mode of cultivation for bioethanol production. Central-Composite Design (CCD) strategy and Response Surface Methodology (RSM) was used for the optimization of microalga growth and ethanol production under laboratory conditions. The results showed that high specific growth rate and biomass productivity of 0.342 day⁻¹ and 0.497 g L⁻¹ day⁻¹ respectively, were achieved at high concentration of phosphate (0.115 g L⁻¹) and pH (9) at 21st day of cultivation. The elemental composition of optimized biomass has shown enhanced elemental accumulation of certain macro (C, O, P) and micronutrients (Na, Mg, Al, K, Ca and Fe) except for nitrogen and sulfur. The Fourier transform infrared spectroscopic analysis has revealed spectral peaks and high absorbance in spectral range of carbohydrates, lipids and proteins, in optimized biomass. The carbohydrates content of optimized biomass was observed as 58%, with 29.3 g L⁻¹ of fermentable sugars after acid catalyzed saccharification. The bioethanol yield was estimated as 51 % g ethanol/g glucose with maximum of 14.9 g/L of bioethanol production. In conclusion, it can be inferred that high specific growth rate and biomass productivity can be achieved by varying levels of phosphate concentration and pH during cultivation of Closteriopsis acicularis for improved yield of microbial growth, biomass and bioethanol production.